For global environmental conservation, it has been desired to use natural energy with a low impact on the environment. As one of natural energy, wind energy is promising. A wind turbine is a rotary machine that converts wind energy into electric energy. As shown in FIG. 1, the wind turbine is composed of a support tower 101, a wind turbine base 102 turnably supported by the support tower, and a wind turbine rotor (rotor head) 103 rotatably supported to the windmill base 102. A plurality of blades (three blades in this example) 104A, 104B, and 104C are turnably supported to the rotor head 103 via slewing bearings 105A, 105B, and 105C (in such a manner that the pitch can be varied), respectively. As shown in FIG. 2, the slewing bearing 105B is composed of a non-rotary outer ring 106 on a rotor head side and a rotary inner ring 107 on a blade side. An annular rolling element row 108 is provided between the outer ring 106 and the inner ring 107. A rolling element as an element of the rolling element row 108 has a shape of a rolling ball or a rolling roller with a substantially cylindrical surface or a spherical surface.
On the slewing bearing 105B supporting the blade 104B as one of the three blades shown in FIG. 1 act an external force Fxb in a radial direction XB; a rotation moment Mxb around the direction XB; an external force Fyb in a radial direction YB; a rotation moment Myb around the direction YB; an external force Fzb in an axial direction ZB orthogonal to the rotation axis of the rotor head 103; and a rotation moment Mzb around the axial direction ZB. Such three-dimensional forces generate a surface pressure against the outer ring 106, the inner ring 107, and a large number of rolling elements included in the rolling element row 108. Such surface pressure acts as an elastic deforming force on the outer ring 106, the inner ring 107, and the rolling element row 108. Such a deforming force is expressed as a distribution function of the circumferential positions corresponding to an element number of each of a large number of rolling elements arranged on the same circumference, and element load imposed on the rolling elements or surface pressure at this position is not constant but greatly variable. Such a deforming force appears as a cause of large friction generated at the slewing bearings 105A, 105B, and 105C, shortening the life of the slewing bearing.
In conjunction with the above description, Japanese Laid Open Patent application (JP-P2002-13540A) discloses a double-row slewing bearing. In this conventional example, an insertion hole is provided for an outer ring or an inner ring in a radial direction and rolling elements are inserted from the insertion hole. This conventional example describes that an amount of pre-load increases gradually as the rollers are inserted, but does not describe the amount of pre-load for each row.
In addition, Japanese Laid Open Patent application (JP-A-Heisei 7-310645) discloses a windmill blade. In this conventional example, a blade section is supported by a rotor head via a slewing bearing in such a manner that the pitch can be varied, and the slewing bearing supports radial load and thrust load at the same time. This slewing bearing is a single-row bearing.